Several fixes for an issue reported by Jonathan Cameron, and other issues found by myself; README improvement (mainly by Jonathan Cameron).

    * error.c:
        * hplibs_error_get(): multiple fixes;
        * hpfiles_error_get(), hpcables_error_get(), hpcalcs_error_get(), hpopers_error_get(): use a single return per function;
    * hpcables.c:
        * hpcables_handle_new(): explicitly initialize handle->handle to NULL;
    * link_prime_hid.c:
        * cable_prime_hid_send(), cable_prime_hid_recv(): return an error if the cable handle is not open;
        * cable_prime_hid_close(): reset handle->handle and handle->fncts to NULL upon successful close;
        * whitespace and comment changes;
    * test_hpcalcs.c:
        * recv_screen(): mention that the main screenshot formats, for the Prime, are 8 to 11.

Signed-off-by: Lionel Debroux <lionel_debroux@yahoo.fr>